Difficulty in understanding the relationship between subnets and physical connections among nodes in the network

Hi! I am new to networking and now I am stuck at understanding the subnets :( I understand that IP addresses are the logical way of addressing computers on the network. And subnets are there to divide the logical address space into smaller ones. Is that right?

Now how are the subnets related to physical network configurations? Suppose there is an Ethernet-based LAN, where all the nodes are connected through... ethernet. Is it necessary that all the nodes should belong to the same subnet? Can there be multiple subnets in the same ethernet network and also can there be multiple ethernets in the same subnet? That is, is it necessary to have a one-to-one relationship between the subnet and physical ethernet connections?

Now if it isn't necessary to have a one-to-one relationship between the two, what happens in case of broadcasts? Suppose on level- 3 of OSI, some device wants the information to be broadcasted to all the devices in the subnet. But the subnet consists of many ethernets. How is the request to broadcast handles on level-2?

On the other hand, suppose the subnet is just a part of ethernet ( that is many subnets in a single ethernet ), now how is the request to broadcast to all members in the subnet handles in level-2?

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PS: Honestly, I am fed up people who just downvote questions without reasons... I ask questions on reddit and not on SO for the very reason that people are nicer here and answer even the noob questions... So if you're gonna downvote the question, at least try to answer it and then downvote.